In case nitrogen is produced from air by an air separation unit, for example, as disclosed in the official gazette of Japanese Patent Application Laid-open No. 225,568/1986, the following method has been hitherto adopted. Namely, after impurities which become catalyst poisons for an oxidation catalyst such as sulfur oxides (SO.sub.x) and hydrogen sulfide (H.sub.2 S) are insufficiently removed from feed air, the feed air is fed to an oxidation catalyst column, where carbon monoxide and hydrogen are oxidized and then removed therefrom by adsorption, and nitrogen gas and oxygen gas are introduced to a nitrogen rectification column, and as a result, a part of nitrogen is taken out as a product and oxygen is used together with the remaining part of nitrogen except for the product as a regenerating gas for an adsorption column and then discharged as an impure gas.
The official gazette of Japanese Patent Application Laid-open No. 177/1992 discloses a nitrogen purifying method and a purifier therefor, in which H.sub.2 is removed by rectification, and the official gazette of Japanese Patent Application Laid-open No. 9,587/1992 discloses also a nitrogen purifying method and a purifier therefor, in which H.sub.2 is separated by rectification, after CO is removed by low temperature adsorption. Furthermore, in the official gazette of Japanese Patent Application Laid-open No. 86,474/1992, an excess amount of oxygen added to oxidize CO and H.sub.2 is exhausted as a high boiling point component.
However, nitrogen obtained by the aforementioned air separating methods in the prior art contains impurities such as O.sub.2, H.sub.2, CO, CO.sub.2, C.sub.n H.sub.m and H.sub.2 O which are undesirable in the manufacture of submicron LSI, in an order of several ppb.
In the methods, in which CO and H.sub.2 are removed through their reactions at a pretreatment step, as disclosed in each of the aforementioned official gazettes, there is such a problem that the activity of a catalyst is remarkably lowered by catalyst poisons such as SO.sub.x and H.sub.2 S in the atmosphere, and there are further such problems that, in the removal of CO through rectification, the recovery efficiency is decreased and a large number of rectifying trays are required, because the vapor pressure of CO is near to that of nitrogen.
The present invention is intended to eliminate the aforementioned drawbacks.